Scholarly Resources for CompSci Undergrads

C Programming Language

Using C to Encapsulate an Abstract Data Type

I used to think that C couldn't be used to create a truly encapsulated data type. I am very pleased to say that I was wrong. One of P. J. Plauger's essays showed me how (see References, below) to do it without incurring any runtime penalty or giving up protoype checking. Although I've written the example using ISO C the same technique can be used with K&R, aka traditional, C (see the Plauger reference for details).

One day I might give a lecture about this technique. In the meantime, I hope the commented example (below) will help someone else to understand how to do this.


This document is composed of three units: an example, some references, and some suggested further reading.


All of the 7 files below in a single tar archive which has been encoded with gzip. It's about 4K long.
The bulk of the encapsulation code is in this file. I made this file from a bigger one that I use for many projects and left in the static open_files variable even though it isn't used in the rest of the example. I think it is instructive.
Contains the function prototypes and definition of the opaque type.
I always find myself defining a few types for whatever project I do. I've taken to putting all those definitions into one file.
Shows how I work with the out-of-date libraries at UWOCSD (using prototypes from the man pages), a nifty error reporting function based on error() from K&R and a driver for the Stream ADT.
Function prototypes for the global functions.
A program with more than one file needs a makefile, doesn't it? :)
A previous version of stream.h that did not use const function pointers.


The links here lead to my comments on these books in my list of recommended technical books.

  1. Programming on Purpose: essays on software design
    by P. J. Plauger
    published by Prentice-Hall, 1993
    pp. 201-2 (part of Essay 22: Inherit It)
  2. C Programming FAQs: frequently asked questions
    by Steve Summit
    published by Addison-Wesley, 1996
    `Question 2.4: What's the best way of implementing opaque (abstract) data types in C?' (p. 32); and
    definition of opaque type (p. 367)

See Also

Bobby Schmidt discussed several ways of encapsulation in the The Learning C/C++urve column of C/C++ User's Journal during 1997. See in particular:

  • Jan 1997 Driving You to Abstraction (pages 65 to 70)
  • Feb 1997 Further Adventures in Abstraction (pages 71 to 77)
  • May 1997 Let Me Say That About this (pages 73 to 77)
  • Jun 1997 Sargasso (pages 77 to 85)
  • NOTE: In April 2019 a watchful surfer informed me that the website at is no longer the homepage of the journal named above. (Thanks `A.S.'!)

    The method I present is discussed in the June column (because I wrote to Bobby Schmidt about it).

    I have a list of various resources for undergraduate computer science students at UWO.

    This document is copyright by its author, J. Blustein. Last modification 01 April 2019 (removed live link to former C/C++ Users Journal homepage); Penultimate modification 19 June 1997 (included references to Bobby Schmidt's columns); Previous modification 31 July 1996.